Structural Controls of Ore Mineralization in a Polydeformed Basement: Field Examples from the
2. Geological and Ore Deposits Outline of Baccu Locci Mine District 1. Regional Geology
The section of Variscan chain exposed in Sardinia consists of three tectono-metamorphic zones: (1) an inner zone in the north, with medium- to high-grade regional metamorphism, thrust on (2) a nappe zone in central-south, with low-grade regional metamorphism, in turn thrust on (3) a foreland zone in the southwest, non-metamorphosed or with very low-grade regional metamorphism (Figure1a,b). The emplacement of the tectonic units is generally from the top to the south, with some exceptions [6]. At the map scale, in the nappe zone a distinction can be made between external nappes in the south, displaying a well recognizable lithostratigraphic succession, and internal nappes in the central-north, where it is not easy to recognize the litho-stratigraphic succession. The study area is part of the external nappe zone (Figure2) and it consists of three tectonic units (from the bottom: Riu Gruppa, Gerrei, and Meana Sardo units) stacked one above the other during the Variscan shortening phases and then affected by late orogenic extension (Figure3).
Figure 1.(a) Geological sketch map of the Variscan basement of Sardinia. (b) Tectonic sketch of the SE Sardinia Variscan basement. (c) Geological cross-section of the Gerrei-Sarrabus region in SE Sardinia;
modified after [7], reprinted with permission.
Figure 2.Tectonic sketch map and mineral deposits of the Gerrei district.
Figure 3.Structural schematic map of the Baccu Locci shear zone and geological cross-section, after [2].
In the geological cross-section, red circles point out D2 folds that are schematically represented in Figure 15.
2.1.1. Stratigraphic Outline
The tectonic units share a similar lithostratigraphic outline (with some differences, especially in the Middle Ordovician volcanic sequences), which consists of four middle Cambrian to lower Carboniferous successions, separated by three large regional unconformities [8]. They are:
• a mainly siliciclastic succession, with rare interlayered volcanic rocks, of middle Cambrian to Early Ordovician age (Arenarie di San VitoFm.);
• a Middle Ordovician volcano-sedimentary succession with tuffites, metavolcanoclastites, and interlayered epiclastites, andesitic in the lower part (M. S. Vittoria Fm.) and rhyolithic in the upper part (“Porfiroidi” Auct.Fm.); the basal contact of this succession is marked by some discontinuous conglomerates (Metaconglomerato di MuraveraFm.);
• a siliciclastic to carbonatic succession of Late Ordovician to early Carboniferous age, with lithic sandstones and arkosic arenites (Metarcose di Genna MesaFm., Late Ordovician), siltstones and marls (Scisti di Rio CanoniFm., Late Ordovician.), black shales and limestones (“Scisti neri a graptoliti” Auct. Fm., Silurian to Early Devonian; “Calcari di Villasalto” Auct. Fm., Early Devonian to early Carboniferous);
• a lower Carboniferous siliciclastic sequence with conglomerates, sandstones, and olistoliths of the older formations that unconformable rests on the Devonian formations, which does not crop out in the study area (Pala Manna Fm., early Carboniferous); it is the youngest formation involved by the Variscan orogeny in Sardinia.
The metamorphic basement of SE Sardinia is then intruded by an upper Palaeozoic (upper Carboniferous to lower Permian) intrusive complex. Lower Permian leucogranitic rocks outcrop close to the study area in the Quirra sector [9]; they belong to a calc-alkaline, ferroan, F-bearing, ilmenite-series intrusion, part of a magmatic suite dated at 286±2 Ma [10]. The entire period of granitoid magmatism is associated with calc-alkaline volcanism [11] and with widespread mafic and felsic diking [12]. Early diking phases are represented by swarms of calc-alkaline (spessartitic) mafic dikes; one of them, crosscutting the same tectonic unit in a nearby area, is dated at 302 Ma [13].
The early EoceneMonte CardigaFm., made up of conglomerates, sandstones, and marls that are deposited in littoral environments lies unconformably on the metamorphic basement and the intrusive complex. In the surrounding areas, the oldest deposits that unconformably cover the Variscan basement are Middle Pennsylvanian continental deposits [14].
2.1.2. Structural Outline
The oldest tectonic features that are recognizable in the Baccu Locci mine district involve the lower Carboniferous rocks and are related to the Variscan orogeny. Indeed, there is no evidence of the pre-Middle Ordovician tectonic structures recognized in adjacent areas [7], as they were probably obliterated by Variscan deformation. The overlap of several early to late Carboniferous deformation events is evidence that a D1 collisional phase with crustal thickening and subhorizontal shortening occurred under ductile conditions and a D2 postcollisional extension with the reactivation of some of D1 structures occurred in the ductile–brittle transition (Figure4) [6,8,15]. The D1 phase is characterized by a general SSW-directed nappe emplacement, regional folding and thrusting, and syntectonic regional lower green-schist facies metamorphism. An exception is represented by the Sarrabus unit, the shallowest nappe of the stack, which crops out south of the study area and it is emplaced from the top to the west [16]. The D1 early shortening structures are large kilometer-scale recumbent isoclinal folds facing southward, with well-developed penetrative axial plane foliation—generally a slaty cleavage produced in lower green-schist facies metamorphism—followed by almost contemporary south-southwest thrusts that produced thick mylonitic belts. Development of such shear zones between the different tectonic units, thick up to several hundred meters, is common in the Variscan basement of Sardinia [17–24]. Among them, the Baccu Locci shear zone is one of the most noteworthy [3]. It can be followed in the field for more than 30 km; the study area is located on its eastern side. The Baccu
Locci shear zone is characterized by widespread and penetrative mylonitic foliation with a mineral assemblage typical of lower green-schist facies metamorphism that is parallel to the large thrust and generally cuts at a low angle the early D1 axial plane foliation [3]. The deformation is highly partitioned;
in the core of the shear zone, it is not possible to recognize the mylonite protolith, although several slices of less deformed rocks have been recognized and mapped [2,3,19]. (Note that, following the choice by [2,3], in Figures3and5we distinguished the mylonite whose protolith is not recognizable from the mylonite whose protolith is still recognizable). At a later stage during the collisional phase, a late-D1 (LD1) N-S shortening event led to development at a regional scale of large, weakly east-plunging upright folds, with axes up to 50 km long that refolded both isoclinal folds and ductile shear zones.
Of these late folds, the main fold is the Flumendosa Antiform [1] (Figures1and2), which runs roughly from WNW to ESE for more than 50 km, folding the D1 nappe stack. In detail, the Flumendosa Antiform consists of some minor order antiforms and synforms with km-size wavelength. One such northern minor order folds crops out in the study area. The LD1 axis is generally east-plunging, and at the hinge zone, a subvertical spaced crenulation cleavage discontinuously developed. Then, the LD1 folds were in turn deformed during the D2 postcollisional extensional phase. The limbs of the LD1 antiforms are deformed by several asymmetric recumbent folds with subhorizontal axial planes and axes parallel to the LD1 limb attitude [1] (Figure4). The D2 folds are overturned away from the antiformal hinge zone [1]. They are often associated with low-angle normal shear zones that allowed for the exhumation of deeper units and enhanced the antiformal structure. They have opposite structural-facing direction in the fold flanks: north-facing in the northern limb and south-facing in the southern limb. Their major order wavelength exposed in the field is about 30 to 40 m. Folds with
“outer” structural facing and low-angle normal faults are interpreted as produced by vertical shortening of steeply inclined bedding and earlier foliations. During exhumation, rocks were progressively carried to shallower structural levels, where brittle behavior became prevalent. Thus, the deformation style changed, and the final stage of postcollisional extension was accommodated by high-angle normal faults [6]. A D3 folding event, with vertical axial planes and axis trend changing from N-S to N 40◦, is also recognized, but it is still not clear whether it could be related to a final stage of postcollisional extension or to the following D4 strike slip faulting [8]. Finally, D4 strike-slip faults affected the exhumed basement but did not involve the Permian to Eocene successions that crop out in the study area. At the Variscan Realm scale, a late strike-slip tectonics is from far recognized. Moreover, is clearly observable in the field that the lower Permian granitoids postdate the D1, D2, and D3 structures, whereas there is less evidence about their relationship with strike-slip tectonics. As we will describe below, LD1 and D2 folds, as well as late faults, played a significant role in controlling the geometry of orebodies.
Figure 4. Schematic relationships between D1, LD1, and D2 structure. Note the development of recumbent D2 folds in the limb of upright LD1 antiform (modified after [1]). The red dashed box indicates the location of the scheme in Figure 15.
Figure 5.Mineralized outcrops in the Baccu Locci mine area, after [2].
2.2. The Gerrei-Sarrabus Metallogenic District
The Gerrei-Sarrabus region has been historically the second most important mining area in Sardinia and the most important antimony district in Italy (Villasalto–Ballao district). Several reviews of the district’s mineral deposits have been done in the past [25–27], including a recent attempt to interpret some of the main mineral associations in the Variscan metallogenic epoch of Sardinia [28].
The main mineral deposits of the region include the following (Figure2):
(a) Zn–Cu–Pb sulphide lenses, disseminations, and ores;
(b) Sb–W, As–Au, and As–Pb–(Cu, Zn, Ag, Au) mesothermal systems with quartz–sulphide veins, stockwork, and disseminated ores;
(c) Mo–W–F greisen and vein-type ores;
(d) F–Ba±Pb–Ag–Cu–Zn vein systems.
Ores of (a) and (b)-types are hosted in the Palaeozoic metamorphic basement, and mostly occur in the northern part of the district (Gerrei); (c) type ores are hosted in and/or related to the suite of F-bearing Permian granites of San Vito and Quirra intrusions [9,29]; and, (d) type ores occur prevalently in the southern part of the district (Sarrabus), are broadly typified by the “Sarrabus Silver Lode” [30] and Silius [31] deposits, and they are possibly related to another suite of F-rich Permian granites (Sette Fratelli and Monte Genis intrusions [9]). Although still very lacking and non-systematic, some isotope and fluid inclusion data on different Gerrei-Sarrabus ore deposits are available from several recent studies [10,25,30–32].
Only (a) and (b) type ores are present in the study area, and thus will be considered in detail in this work. Both types are structurally controlled and are located at different structural levels.